This invention relates to a low operating current longwavelength region quantum barrier semiconductor optical device superior in temperature characteristic.
Well known semiconductor laser diodes (LD) for long wavelength region optical communications are double heterostructure LDs with a GaInAs(P) active layer and an InP clad layer, which are put in practical use as light sources for trunk optical communication lines.
Longwavelength region laser devices are fitted with a Peltier device when applied to trunk line systems since they are inferior to shortwave length region ones in temperature stability.
Lasing threshold current I.sub.TH of laser diodes is expressed as function of temperature T by equation: EQU I.sub.TH.=I.sub.THO X exp (T/T.sub.o) (1)
wherein T, T.sub.o represent operating temperature and characteristic temperature in Kelvin respectively. As is seen in eq(1) , high T.sub.o values mean superior stability of threshold current in wide temperature range. For example, shortwavelength region laser diodes exhibit characteristic temperature T.sub.o &gt;150K. In the case of longwavelength region laser diodes, on the other hand, their characteristic temperature T.sub.o marks 130.degree.-140.degree. K below the temperatures less than approximately 10.degree. C. but lowers to 60.degree.-80.degree. K above the temperatures approximately 10.degree. C.
The following three mechanisms have been considered as causes of low characteristic temperature of longwavelength region laser diodes.
1-1 Thermal excitation and diffusion leakage of electrons from the active layer to the clad layer. PA1 1-2 Leakage of hot electrons excited in the conduction band by Auger CHCC process and over flow to the clad layer. PA1 1-3 Increase of light absorption due to the excitation of holes in valence band by Auger CHHS process. PA1 2-1 Their threshold current densities are about twice as large as those of conventional semiconductor laser diodes. PA1 2-2 When the thicknesses of active and middle clad layers are decreased, not only is the threshold current density decreased but also characteristic temperature approach the values of conventional semiconductor laser diodes.
Of the above, mechanism 1-2 is said to be a main cause of lowering the characteristic temperature in longwavelength region laser diodes.
The reasons for regarding 1-2 as main cause of lowering characteristic temperature T.sub.o, (K) are as described in the article by: G. P. Agrawal et al.: Longwavelength Semiconductor Lasers, Van Nostrand Reinhold Company, pp. 70-138. This article describes that the leakage current increment via mechanism (1-1) is as small as less than 100 A/cm.sup.2 in the 1.3 .mu.m region and that light absorption increase via mechanism (1-3) does not sufficiently elucidate actual dependence of current on temperature.
How to decrease the electron overflow caused by the Auger effect is therefore of key factor in improving temperature characteristics of longwavelength region laser diodes.
From this standpoint, use of DCC (double carrier confinement) and MQB (multi-quantum barrier) methods have been studied to improve temperature characteristic of longwavelength region laser diodes.
The article by M. Yano et al. published in the IEEE J. Quantum Electron. Vol. QE-19, pp. 1319-1327, describes DCC type laser diodes as shown in FIGS. 4(a) to (c) with cross sectional structure, energy band diagram and temperature characteristics of threshold current.
The DCC structure in FIG. 4(a) is fabricated by growing and forming n-Inp clad layer 2, a first GaInAsP active layer 3, a p-InP middle clad layer 4, a second p-GaInAsP active layer 5, a p-InP clad layer 6, a p-GaInAsP contact layer 7, an n-InP block layer and a p-electrode 9 on an n-InP substrate 1.
The laser diodes with such a DCC structure as shown in FIG. 4(a) show improved characteristic temperature of 130.degree. to 210.degree. K up to about 80.degree. C. as is shown in FIG. 4(c) and also exhibit reduced temperature dependency on the differential quantum efficiency.
The reason for improved temperature characteristics of DCC structure LD is that the hot electrons overflowing from the first active layer generating the main part of the gain required for lasing are scattered by the middle clad layer to lose energy and captured by the second active layer to contribute again to stimulated emission. Therefore, the DCC structure in FIG. 4(a) produces the effect equivalent to the reduction of the effects caused by mechanism 1-2.
In MQB type devices, super lattice structures are formed so as to reflect electrons as waves in such phase that the reflected and incident waves enhance each other i.e. to realize the maximum value of the reflection coefficient of incident electron waves.
Such MQB type devices are dealt with in the Japanese Laid-Open Patent Official Gazette No. 46788/1988 (Iga, Koyama and Uenohara, Tokyo Institute of Technology) and the article by Iga, Uenohara and Koyama, Electronics Letters Vol. 22 pp. 1008-10091, 1986.
The laser diode mentioned in the Japanese patent publication describes an MQB electron reflecting layer 10 between GaInAsP active layer 3 and a P-INP clad layer 6 as schematically shown in FIG. 5.
Specifically, effective barrier height of the laser diode in FIG. 5 is made higher than a classical one by providing a plurality of periodic structures with different barrier thicknesses and well thicknesses between its GaInAsP active layer 3 and a P-INP clad layer 6.
Recently, MQB type visible red laser diodes have been described by K. Kishino et al. , IEEE, Laser Conference, PD-10, 1990 The literature states that characteristic temperature is increased and threshold current density is reduced when an GaInP/AlInP MQB structure is introduced. T. C. Hasenberg et al., Applied Physics Letter, Vol. 49 No. 7, p. 400, 1986, states the following problems to be solved for DCC structures.
Problem 2-2 is attributed to the mechanism in which energy relaxation activity at the middle clad layer decreases so as electrons become difficult to be captured by the second active layer.
Conventional MQB devices proposed so far pose such problems as described below.
Theoretical study as to the increase of effective barrier height in MQB structure in materials for longwavelength region optical devices is discussed by Uenohara, et al., The Transactions of the Institute of Electronics, Information and Communication Engineers Vol. J70-C No. 6 pp. 851-857, 1987. This article states that effective barrier height of Ga.sub.0.47 In.sub.0.53 As/InP MQB laser devices can be increased by 0.16 eV in height. Therefore, resulting effective total barrier height is estimated to be 0.5 eV by adding conventional hetero barrier height 0.35 eV.
But the main cause of low characteristic temperature in longwavelength region laser diodes is considered to be overflow of the hot electrons generated by the Auger CHCC process. And the maximum energy level of those hot electrons is about 1 eV higher than the conduction band edge, therefore 0.5 eV higher than the above mentioned effective barrier height 0.5 eV, for 1.3 .mu.m LDs. However, no report has been published on any MQB laser devices effective for such hot electrons, so far.
In view of what is described heretofore, principles of MQB structures will be briefly discussed with the simplified model of one dimensional collision problem of an electron wave with one well potential or one barrier potential made by L. I. Schiff, Quantum Mechanics, P. 100, Continuous Eigenvalues: Collision theory, McGraw-Hill.
In FIG. 6(a) , symbol V.sub.o denotes the well depth, E.sub.o, energy of a incidence carrier, (m) effective mass of the carrier in the well and (a), thickness of the well.
In this case, reflection coefficient R is given by following equation. EQU R=1/{1+[4E.sub.o (E.sub.o +V.sub.o)]/[V.sub.o sin.sup.2 (k.sub.2 a)]}(2)
where k.sub.2 is given by: EQU k.sub.2 ={[2m(E.sub.o +V.sub.o)]/(h/2.pi.).sup.2 }.sup.1/2 ( 3)
The phase condition for giving the maximum value R.sub.max of reflection coefficient is given by following equation. EQU k.sub.2 a=[n+(1/2)].pi. (4)
where, n=0, 1, 2 . . . At this condition, R.sub.max is given by equation: EQU R.sub.max =1/{1+[4E.sub.0 (E.sub.o +V.sub.o)]/(V.sub.o.sup.2)}(5),
representing collision of an electron wave with one dimensional barrier potential.
In FIG. 6(b), symbol V.sub.o denotes barrier height, E.sub.o energy of a incident carrier, (m), effective mass of the carrier in the barrier and (a), thickness of the barrier. When E.sub.o .gtoreq.V.sub.o, the reflection coefficient R mentioned above is expressed by equation: EQU R=1/{1+[4E.sub.o (E.sub.o -V.sub.o)[/]Vo sin.sup.2 (k.sub.2 a)]}(6)
where, EQU k.sub.2 ={[2m(E.sub.o -V.sub.o)]/(h/2.pi.).sup.2 }.sup.1/2 ( 7)
The phase condition for giving the maximum value R.sub.max of reflection coefficient is given by equation: EQU k.sub.2 a=[n+(1/2)].pi. (8)
where, n=0, 1, 2. At this condition, R.sub.max is given by equation: EQU R.sub.max =1/{1+[4E.sub.o (E.sub.o -V.sub.o)]/(V.sub.o.sup.2)}(9)
When V.sub.o &gt;E.sub.o &gt;0, the reflection coefficient R mentioned above is expressed by equation: EQU R=1/({1+[4E.sub.o (V.sub.o -E.sub.o)]/[V.sub.o.sup.2 sinh.sup.2 (k.sub.1 a)]} (10)
and (k.sub.1) therein is given by equation: EQU k.sub.1={[2 m(V.sub.o -E.sub.o)]/(h/2.pi.).sup.2 }.sup.1/2 ( 11)
In this case, therefore, there is no resonance condition, so the reflection coefficient R comes close to "1" with the increase of value (k.sub.2 a) when width (a) of the barrier is increased.
The MQB laser diodes which require coherency of electron wave are subject to the limitation of increasing thickness of barrier because coherent length is limited by the order of the mean free path of the carrier.
The energy level of hot electron with maximum energy corresponds to equations (5) or (9) in case of well or barrier potential respectively. Therefore, R.sub.max values were calculated as functions of normalized incident energy (E.sub.o /V.sub.o) for equations (5) and (9), obtaining the results as shown in FIGS. 7(a) and 7(b).
As is apparent from FIGS. 7(a) and 7(b), the maximum value R.sub.max of the reflection coefficient R decreases when value (E.sub.o /V.sub.o) increases.
FIG. 8 shows the potential configuration of a barrier made of InP and Al.sub.x In.sub.1-x As (x=0.48) said to have the smallest electron affinity among materials lattice matched to InP. In the case of such barrier potential as shown in FIG. 8, an AlInAs layer 11 lattice matched to InP is provided between p-InP clad layer 6 and GaInAsP active layer 3.
When this barrier potential is applied to a 1.3 .mu.m LD, hot electron with maximum 950 meV gives E.sub.o /V.sub.o =2.93. The corresponding R.sub.max is 0.04 as is clear in FIG. 7(b).
That is, this barrier potential reflects only 4% of hot electrons with maximum energy even under the phase condition of maximum reflection.
When a multi-layer barrier structure is used to obtain reflection factor R=1, more than 10 barrier layers are necessary for a specific incident energy. A MQB layer which is more than several times as thick as the above multi-layer barrier is necessary for its application to a wide range of incident energy.
This is not desirable for aforementioned coherent length because it lowers coherency and decreases its effect as an MQB.
As is stated heretofore, the potentials made from materials lattice matched to InP are not suitable for MQB layers, having no adequate function of reflecting the hot electrons generated due to Auger CHCC process.